The invention relates to a heating module for an exhaust-gas purification system connected to the outlet of an internal combustion engine. The heating module comprises a catalytic burner, an HC injector and an oxidation catalytic converter positioned downstream of the HC injector in the flow direction of the exhaust gas. The oxidation catalytic converter supplies thermal energy to an exhaust-gas purification unit of the exhaust-gas purification system. The heating module has a main section, a secondary section which comprises the catalytic burner, and a device for controlling the exhaust-gas mass flow flowing through the secondary section.
Internal combustion engines today, diesel engines in particular, comprise control units that are connected in the exhaust gas system in order to reduce harmful or undesired emissions. Such a control unit can be, for example, an oxidation catalytic converter, a particle filter and/or a selective catalytic reduction (SCR) stage. A particle filter is used to collect soot particles discharged by the internal combustion engine. The soot that is present in the exhaust gas accumulates on the upstream side surface of the particle filter. In order to prevent an excessive increase in the exhaust gas counter pressure during the course of the successive soot accumulation and/or to prevent the risk of clogging the filter, a regeneration process is triggered when the soot load of the particle filter reaches a sufficient level. In such a regeneration process, the soot that accumulates on the filter is burnt off (oxidized). After the completion of such a soot oxidation, the particle filter is regenerated. Only a noncombustible ash residue remains. For a soot oxidation to occur, the soot must be at a certain temperature. As a rule, this temperature is approximately 600° C. The temperature at which such a soot oxidation starts can be lower, for example, if the oxidation temperature has been reduced by an additive or by providing NO2. If the soot is at a temperature which is below its oxidation temperature, then thermal energy is required to trigger the regeneration process. An active regeneration can be started using engine-internal measures, by changing the combustion process so that the exhaust gas is discharged at a higher temperature. In numerous applications, particularly in the non-road field, post-engine measures are preferable in order to produce an active regeneration. In many cases, it is not possible in the context of exhaust emission control to have an influence on the engine-based measures.
DE 20 2009 005 251 U1 discloses an exhaust emission control unit, wherein, for the purpose of actively producing the regeneration of a particle filter, the exhaust gas system is divided into a main exhaust gas system and a secondary exhaust gas system. These two systems form a heating module. A catalytic burner is connected in the secondary system. The catalytic burner heats and subsequently merges the partial exhaust gas flow flowing through the secondary system with the partial exhaust gas flow flowing through the main system. In this manner, the mixed exhaust gas mass flow is at a clearly higher temperature. The increase in the temperature of the exhaust gas flow heats the soot accumulated on the upstream side of the particle filter to a sufficient temperature to trigger the regeneration process. An oxidation catalytic converter having an upstream hydrocarbon injection, which is located in the secondary system, is used as catalytic burner. An exhaust flap controls the exhaust gas mass flow flowing through the secondary system. The exhaust flap sets the cross-sectional area that allows free flow in the main system. An electrothermal heating element is connected upstream of the oxidation catalytic converter. The electrothermal heating element heats the oxidation catalytic converter to its light-off temperature—namely the temperature at which the desired exothermic HC conversion starts to occur on the catalytic surface. The electrothermal heating element is activated when the oxidation catalytic converter has to be heated to its light-off temperature. This document also describes that the catalytic burner connected in the secondary system can be oversprayed in order to feed hydrocarbons to a second oxidation catalytic converter directly upstream of the particle filter, so that these hydrocarbons can react with the same exothermic reaction on the catalytic surface of this second oxidation catalytic converter. In this manner, a two-step heating of the exhaust gas can be carried out in this previously known emission control installation. The exhaust gas flowing out of the second oxidation catalytic converter is then at the required temperature in order to heat the soot accumulated on the upstream side of the particle filter sufficiently so that the soot oxidizes.
Similarly, it can be desirable to increase the temperature of other exhaust emission control units, for example, of an oxidation catalytic converter or of an SCR stage, in order to bring the latter more rapidly to their operating temperature.